switch approver and post pruners to use system user

app/logical/approver_pruner.rb

@@ -7,10 +7,8 @@ class ApproverPruner
   end
 
   def prune!
-    admin = User.admins.first
-
     inactive_approvers.each do |user|
-      CurrentUser.scoped(admin, "127.0.0.1") do
+      CurrentUser.scoped(User.system, "127.0.0.1") do
         next if user.is_admin?
 
         janitor_trial = JanitorTrial.where(user_id: user.id).first

app/logical/post_pruner.rb

@@ -1,10 +1,4 @@
 class PostPruner
-  attr_reader :admin
-
-  def initialize
-    @admin = User.where(:level => User::Levels::ADMIN).first
-  end
-
   def prune!
     Post.without_timeout do
       prune_pending!
@@ -16,7 +10,7 @@ class PostPruner
 protected
 
   def prune_pending!
-    CurrentUser.scoped(admin, "127.0.0.1") do
+    CurrentUser.scoped(User.system, "127.0.0.1") do
       Post.where("is_deleted = ? and is_pending = ? and created_at < ?", false, true, 3.days.ago).each do |post|
         begin
           post.flag!("Unapproved in three days")
@@ -29,7 +23,7 @@ protected
   end
 
   def prune_flagged!
-    CurrentUser.scoped(admin, "127.0.0.1") do
+    CurrentUser.scoped(User.system, "127.0.0.1") do
       Post.where("is_deleted = ? and is_flagged = ?", false, true).each do |post|
         if post.flags.unresolved.old.any?
           begin
@@ -44,6 +38,6 @@ protected
   end
 
   def prune_mod_actions!
-    ModAction.destroy_all(["creator_id = ? and description like ?", admin.id, "deleted post %"])
+    ModAction.destroy_all(["creator_id = ? and description like ?", User.system.id, "deleted post %"])
   end
 end

app/models/user.rb

@@ -301,6 +301,10 @@ class User < ActiveRecord::Base
     extend ActiveSupport::Concern
 
     module ClassMethods
+      def system
+        Danbooru.config.system_user
+      end
+
       def level_hash
         return {
           "Member" => Levels::MEMBER,